The present invention relates to a pneumatic tire. More specifically, the present invention relates to a pneumatic tire in which wet performance degradation associated with wear progress is reduced.
Conventionally, a pneumatic tire in which blocks are defined by grooves in the tread surface has sipes provided in the blocks in order to improve drainage performance. A water film formed on a wet road surface is cut by using the edge effect of the sipes, and the blocks are brought into contact with the road surface on which the water film has been cut, whereby grip is improved. However, since the volume of the grooves decreases with the wear progress of the blocks, wet performance degradation cannot be avoided.
In this connection, Japanese patent application Kokai publication Nos. 2-246810 and 9-183303, for example, disclose pneumatic tires having sipes, each having a length that gradually increases toward the bottom of the sipe in order to reduce wet performance degradation in a worn condition. Since the length of the sipes exposed on the blocks increases with wear progress, the edge effect of the sipes increases in a worn condition. This reduces wet performance degradation.
However, in a pneumatic tire having sipes of which at least one ends do not communicate with grooves but are located within blocks to improve block rigidity, if the sipe length at a position on the bottom side of the sipe is made longer than that at a position on the block surface side thereof, the release characteristics of the blades of a mold for forming the sipes is poor, and cracks are prone to appear in the sipe walls at the sipe end portions. Furthermore, since the block rigidity in an unworn brand-new condition is lower than that of a tire in which the sipe length is constant up to the sipe bottom, wet performance in a brand-new condition is degraded.